Method for the production of plastic moulded parts

ABSTRACT

A method is described for producing fibre-reinforced plastic moulded parts, wherein endless fibre strands are fed by a fibre braking device and/or cut fibres via a gravimetric metering device, and a plastic material to be melted, by a volumetric metering device are fed to a single-screw plasticizing unit. Plastic material which is molten and is mixed with fibre material is injected into a moulding tool by an injection stroke of the plasticizing screw. According to the application, the ACTUAL mass flow of the plastic material is calculated from the ACTUAL volume flow of the plastic material and from the ACTUAL mass flow of the fibre material. The ACTUAL mass flow of the plastic material is compared to a TARGET mass flow of the plastic material, and the rotation speed n d  of the rotary drive of the metering element of the metering device is adapted.

The invention relates to a method for the production of plastic mouldedparts according to the introductory clause of Claim 1.

WO 2011/066917A2 discloses an injection moulding machine for theproduction of fibre-reinforced plastic moulded parts, with a cylinderand with a plasticizing screw that is able to be driven rotatably andlinearly in the cylinder, wherein in the cylinder a first opening isprovided as filling opening for the feed of a plastic material which isto be melted, and wherein on the conveying remote side from the firstopening in the cylinder, a second opening is provided as filling openingfor the feed of one or more fibre bundles. The fibre bundles can bedrawn off from one or more fibre spools. In the production of afibre-reinforced plastic moulded part, the fibres are drawn in from theplasticizing screw on its rotation and are mixed into the melt. In sodoing, the fibre bundles run through a fibre braking device. The plasticmaterial which is to be processed is fed as granulate, wherein agravimetric or a volumetric metering device can be provided.

In volumetric metering, the discharge of the granulate takes placeexclusively in a volume-related manner. The metering elements of avolumetrically operating metering device are to be calibrated to therespective material, i.e. it is to be determined how much material themetering element doses in a defined period of time. As metering elementfor example a metering screw can be provided. A problem in volumetricmetering devices is that fluctuations in the bulk density can not becompensated automatically. Such fluctuations in the bulk density of thegranulate lead, however, to a change in the ratio of fibre material andof plastic material in the injection-moulded, fibre-reinforced plasticmoulded parts. In the production of fibre-reinforced plastic mouldedparts it is desirable, however, that as constant a ratio as possible offibre material and of plastic material is maintained during theproduction of the fibre-reinforced plastic moulded parts.

In gravimetric or weight-regulated metering, one or more load cellsintegrated in a suitable manner into the metering device measure i.e.weigh the granulate which is to be metered. By means of the weight asmeasured value, a regulating of the metering can take place throughtarget/actual comparison. Metering devices operating in a gravimetricmanner can therefore automatically compensate fluctuations in the bulkdensity. A disadvantage in gravimetrically operating metering devices isthat they are distinctly more expensive than volumetric meteringdevices, in particular owing to the use of highly sensitive load cellsand a relatively complex control unit for the operation of thegravimetric metering device.

Proceeding from the prior art named in the introduction, the inventionis based on the problem of indicating a method by which plastic mouldedparts can be produced on an injection moulding machine which is equippedwith a single-screw plasticizing unit, wherein a volumetric meteringdevice can be used for the plastic material and nevertheless as constantan ACTUAL mass flow of plastic material as possible can be maintained.

The solution to this problem takes place through a method with thefeatures of Claim 1.

A further problem forming the basis of the invention can be seen inindicating a method by which fibre-reinforced plastic moulded parts canbe produced on an injection moulding machine equipped with asingle-screw plasticizing unit, wherein a volumetric metering device canbe used for the plastic material, and nevertheless a constant ratio offibre material and of plastic material can be maintained in the finishedfibre-reinforced plastic moulded parts during the production of theseparts.

The solution to this further problem takes place through a method withthe features of claim 2. Advantageous embodiments and furtherdevelopments are to be found in the further claims.

Through the fact that the ACTUAL mass flow of the plastic material iscalculated, wherein the ACTUAL mass flow is calculated from the returnspeed v_(screw,back,n) of the plasticizing screw during a melt meteringprocess, the diameter of the plasticizing screw and the melt density,that the ACTUAL mass flow of the plastic material is compared with aTARGET mass flow of the plastic material, and that with apredeterminable difference value between ACTUAL mass flow of the plasticmaterial and the TARGET mass flow of the plastic material, the rotationspeed of the rotary drive of the metering element is changed in such away that the difference value is reduced, wherein the difference valueis preferably to reach zero, a readjustment of the ACTUAL mass flow ofplastic material can take place and therefore fluctuations in the bulkdensity can be reacted to. In the case of fluctuations in the bulkdensity of the plastic material therefore a readjustment of the meteringcapacity, which corresponds to the ACTUAL mass flow of plastic material,can be carried out and the ACTUAL mass flow can be kept constant. Avolumetric metering device is therefore used, but for the methodaccording to the invention the mass flow of plastic material, which isfed into the cylinder from the metering device, is used and not thevolume flow. A readjustment of the metering capacity of the volumetricmetering device therefore takes place as a result of a calculation ofthe ACTUAL mass flow of the plastic material fed into the cylinder fromthis metering device, and of a comparison of this ACTUAL mass flow witha predeterminable TARGET mass flow.

“Melt metering process” is to be understood here to mean the process inwhich the plasticizing screw mixes, through a rotary movement, plasticgranulate and/or further components, which are added through one or moreopenings in the plasticizing cylinder, converts the mixture into amolten state and conveys the mixture at the conveying remote end of theplasticizing screw into the so-called screw pre-chamber.

Through the pressure occurring in the screw pre-chamber, theplasticizing screw is displaced during the melt metering process alongits axis in the direction opposed to the conveying direction.

The melt density ρ_(s) is a value which depends significantly on thetype of material, the melt temperature and the pressure with which themelt is acted upon. To characterize types of material, compression testsare therefore carried out by material manufacturers and so-called pvTcurves are recorded. In these curves, the specific melt volume v_(s),which represents the reciprocal of the density ρ_(s) (v=1/ρ) is enteredas a function of the present temperature T and the pressure p. In theknown context, v_(s)(p,T), the melt density ρ_(s) can be determined as afunction of the environmental conditions, as follows:

ρ_(s)(p,T)_=1/[v _(s)(p,T)]

ρ_(s)(p,T): melt density

p: pressure

T: temperature

v_(s): specific melt volume

Thereby, it becomes possible that the desired ACTUAL mass flow ismaintained even in the case of plastic material which is critical withregard to dwell time. In particular, in a preferred embodiment the screwcan be operated in an underfed manner. An underfed state designates aform of operation in which less material is delivered to the screw thanthis would draw in from a full hopper. A preferred field of applicationis the production of plastic moulded parts for optical purposes. Plasticmoulded parts for optical purposes, for example lenses, are mostlythick-walled and require a sufficiently long cooling in the mouldingtool. So that during the cooling phase, the plastic present in a moltenmanner in the plasticizing unit does not undergo any damage throughpolymer chain degradation or oxidative degeneration, the reduction ofthe average dwell time of the plastic melt in the plasticizing unit isaimed for. The plastic volume present absolutely in the plasticizingunit is, inter alia, definitive for the average dwell time. As this isreduced through an underfed operating state compared to the materialfeed from a full hopper, the mean dwell time can be reduced through anunderfed operation, and therefore the damage to the plastic melt can becounteracted.

A further field of application relates to the production offibre-reinforced plastic moulded parts. Here, a fibre material can beadded into the cylinder and a fibre-reinforced plastic moulded part canbe produced, wherein endless fibre strands can be fed to theplasticizing unit via a fibre braking device, and/or cut fibres can befed via a gravimetric metering device. Preferably, through a firstopening in the cylinder the plastic material which is to be melted canbe fed as granulate into the cylinder. On the conveying remote side fromthe first opening, the endless fibre strands can be fed via a secondopening, and/or the cut fibres can be fed via a third opening, into thecylinder and can be drawn in by the plasticizing screw through rotation.Plastic material which is molten and is mixed with fibre material can beinjected into a moulding tool through an injection stroke of theplasticizing screw, and a fibre-reinforced plastic moulded part can beproduced. Thereby it becomes possible that also with a use of avolumetric metering device for the plastic material as constant a ratioof fibre material and of plastic material can be maintained during theproduction of the fibre-reinforced plastic moulded parts.

According to a further embodiment, the ACTUAL volume flow of the plasticmaterial can be determined from the volume of plastic material which ismolten and mixed with fibre material, conveyed into the screwpre-chamber during a melt metering process. The ACTUAL mass flow dm/dtof fibre material with known fibre feed speed v_(f) and known fibrestrand thread fineness n_(tex) and the fibre strand number n_(f) can becalculated with the following formula:

dm _(f) /dt=v _(f) *n _(f) *n _(tex)

dm_(f)/dt: actual mass flow of fibre material

v_(f): fibre feed speed

n_(f): number of fed fibre strands

n_(tex): thread fineness of a fed fibre strand

According to a further development of the method according to theinvention, a change to the metering rotation speed n_(d) can be carriedout from injection moulding cycle to injection moulding cycle. However,it is also possible to use a PI controller for a change to the meteringrotation speed n_(d).

According to a further embodiment, the ACTUAL mass flow can be averagedover several injection moulding cycles. The mean value which is thusformed can then be used for a change of the rotation speed n_(d) of therotary drive of the metering drive.

According to a particularly preferred further development of the methodaccording to the invention, the ACTUAL mass flow of plastic material,dm_(k)/dt can be calculated as follows:

dm _(k) /dt=D _(s) *π*v _(screw,back,n)*ρ_(s)(p,T)−dm _(f) /dt

dm_(k)/dt: actual mass flow of plastic material

D_(s): screw nominal diameter

v_(screw,back,n): screw return speed during the plasticizing

ρ_(s)(p,T): melt density

p: pressure

T: temperature

dm_(f)/dt: actual mass flow of fibre material

If for an injection moulding cycle n the ACTUAL mass flow of the plasticmaterial is calculated, an adaptation of the ACTUAL mass flow to theTARGET mass flow can be carried out for one of the subsequent injectionmoulding cycles, in particular for the immediately following injectionmoulding cycle n+1, by changing the rotation speed of the rotary driveof the metering element.

Depending on the granulate which is to be processed, a metering screw ora metering disc can be used as metering element.

Preferably, the endless fibre strands can be drawn off from a fibre gateequipped with fibre spools.

The fibre braking device can preferably be arranged between the fibregate which is equipped with the fibre spools, and the plasticizing unit,and can impart an adjustable fibre conveying speed to the fibre strandswhich are fed to the plasticizing unit. This speed can not be exceeded,even when the screw which is present in the plasticizing unit rotateswith a circumferential speed which is greater than the set fibreconveying speed.

The cut fibres can be delivered as chopped glass fibres or as acomponent of a further plastic granulate. Furthermore, the possibilityexists to cut and then deliver endless fibre strands.

Furthermore, provision can be made to work both endless fibres and alsochopped glass fibres and/or fibre-reinforced granulates into the melt.This can be expedient in particular in the processing of recycledmaterials.

The invention is to be explained further below with the aid of exampleembodiments and with reference to the figures. There are shown:

FIG. 1 First embodiment of an injection moulding machine with asingle-screw plasticizing unit according to the invention;

FIG. 2 Second embodiment of an injection moulding machine with asingle-screw plasticizing unit according to the invention.

FIG. 3 Third embodiment of an injection moulding machine with asingle-screw plasticizing unit according to the invention

FIG. 1 shows the use of a single-screw plasticizing unit according tothe invention as component of an injection moulding machine. Theinjection moulding machine 1 illustrated in FIG. 1 comprisessubstantially a clamping unit 2, indicated only diagrammatically here,and a single-screw plasticizing unit 3 according to the invention. Theclamping unit 2 and the single-screw plasticizing unit 3 are arranged ina manner known per se on a machine bed, which is not illustrated here.The single-screw plasticizing unit 3 comprises a cylinder 4 with aplasticizing screw 5. On the outer side of the cylinder 4, severalheating elements 19 are arranged. The rear end of the screw 5 isoperatively connected with a rotary drive 6 and with a linear drive 7.By means of the linear drive 7, the screw 5 can be moved axially in thecylinder 4, i.e. the screw 5 is configured as a reciprocating screw andis provided for the injecting of plastic melt, mixed with fibres, intoan injection moulding tool, not illustrated here, situated in theclamping unit 2. In the rear end region of the screw threads, a firstopening is provided as filling opening 8 for the feeding of a plasticmaterial which is to be melted.

The plastic material is present as granulate and is delivered to thefilling opening 8 by a volumetric metering device 30. The meteringdevice 30 comprises a storage container 31 to receive granulate, arotatable metering element 32 and a rotary drive 33 for actuating themetering element. The fact that plastic material is used in the form ofgranulate is to be indicated by the dots in the storage container 31.The reference number 34 is given for one of the dots.

On the conveying remote side from the first opening 8, a second openingis provided as filling opening 9 in the cylinder 4 for the feeding of afibre material. The fibre material is preferably introduced via a fibrebraking device 40 into the opening 9 in the form of fibre bundles 10a-10 f, separated spatially from one another. A fibre bundle can also bedesignated as a roving. At the front end, the screw 5 has a backflowbarrier 11, and has at the conveying remote side from the backflowbarrier 11 a mixing part 12 connected in a rotationally fixed mannerwith the screw 5 and corrugating with the latter. FIG. 1 shows asituation as is present at the end of an injection process. The screw 5is situated in its front end position. The conically tapering head ofthe mixing part 12 lies seated in a fitting conical recess in thecylinder 4.

The feeding of the fibre material and the mode of action of the fibrebraking device 40 is to be described in further detail with the aid ofthe fibre bundle 10 a. For a better overview, the fibre feed device,designated as a whole by reference number 13 and only illustrateddiagrammatically, is illustrated on a greatly enlarged scale in relationto the plasticizing unit 3. The fibre feed device 13 comprises a fibrestorage container 14 with one or more fibre spools 15, from whichrespectively a fibre bundle can be drawn off. In the present exampleembodiment according to FIG. 1, a total of six (6) fibre bundles 10 a to10 f are to be fed to the screw 5, so that in the fibre storagecontainer 14 consequently six (6) fibre spools are provided. Only onesingle fibre spool 15 for the fibre bundle 10 a is illustrated here. Thefibre bundle 10 a is guided through a tube 16, configured as anantistatic tube, which has an inlet opening 16 a and an outlet opening16 b. The inlet opening 16 a is arranged at a suitable location at or inthe fibre storage container 14. The fibre braking device 40 is arrangeddownstream of the outlet end 16 b.

The fibre braking device designated as a whole by reference number 40permits the determining of the ACTUAL mass flow of fibre material. Thefibre braking device 40 comprises substantially at least one deflectorroller 17 and at least one brake roller 18, driven in a braking manner,wherein endless fibres are guided in a slip-free manner via bothrollers. Through the slip-free guidance, the fibre feed speed v_(f) canbe determined from the rotation speed of the brake roller n_(bw).

When the fibre bundle 10 s is caught by the screw 5 so that through therotation of the screw 5 the fibre bundle 10 a is drawn into the melt andis thereby withdrawn from the fibre spool 15, the fibre feed device 13acts as a brake, wherein the braking effect and thereby the brakingforce is distributed to the various components of the fibre feed device13 as described below.

A first braking force is provided on the fibre spool 15. The fibre spool15 is rotatably mounted and is braked by the friction of the mounting(not illustrated) in its rotation so that the fibre bundle 10 a isprestressed with approximately 10 Newton.

A second braking force is achieved by means of the tube 16, wherein thetube 16 is installed such that it has one or more circular segments.This leads to a second braking effect with a second braking forcethrough the effect of rope friction in accordance with Euler-Eytelwein.This second braking force generates approximately 70 Newton of the fibredrawing-in force, so that the fibre drawing-in force at the end of thetube 16 is in total approximately 80% of the fibre drawing-in force.

A third braking force is generated by means of the brake roller 18. Thefibre bundle 10 a is guided around a freely rotatably mounted deflectorroller 17 and subsequently around a speed-regulated brake roller 18 andfrom there to the screw 5. The drive of the brake roller 18 takes placepreferably by means of a gear motor. By means of the brake roller 18,the final 20% of the pre-stressing force is applied. The arrangement ofthe deflector roller 17 and brake roller 18 is such that both thedeflector roller 17 and also the brake roller 18 will rotaterespectively with 180°.

The ACTUAL mass flow of the fed fibre material is known from theconditions of the fibre braking device 40 and can be determined asfollows:

dm _(f) /dt=v _(f) *n _(f) *n _(tex)

dm_(f)/dt: actual mass flow of fibre material

v_(f): fibre feed speed

n_(f): number of fed fibre strands

n_(tex): thread fineness of a fed fibre strand

With reference to the volumetric metering device 30, firstly acalibration to the material which is used is to be carried out. Thiscalibration provides an initial metering capacity P_(D,0), whichcorresponds to the granulate throughput in grams per rotation of themetering element 32.

In the following production operation, the ACTUAL mass flow of plasticmaterial of an injection moulding cycle n can be determined as follows:

dm _(k) /dt=D _(s) *π*v _(screw,back,n)*ρ_(s)(p,T)−dm _(f) /dt

dm_(k)/dt: actual mass flow of plastic material

D_(s): screw nominal diameter

v_(screw,back,n): screw return speed during the plasticizing

ρ_(s)(p,T): melt density

p: pressure

T: temperature

dm_(f)/dt: actual mass flow of fibre material

A comparison of TARGET mass flow and ACTUAL mass flow or respectively ofTARGET metering capacity and ACTUAL metering capacity provides aspecification for the adaptation of the rotation speed n_(d) of therotary drive for the metering element, in particular a metering screw.This adaptation can take place for example from cycle to cycle. However,a PI controller can also be used.

Furthermore, it is possible to average the ACTUAL mass flow of plasticmaterial or respectively the ACTUAL metering capacity over severalcycles and to carry out an adaptation of the rotation speed n_(d) on thebasis of this mean value.

By means of the method according to the invention, it is possible to usevolumetric metering devices and to thereby save costs, because these aredistinctly more reasonably priced than gravimetric metering devices.Nevertheless, fluctuations in the bulk density can be detected andcompensated. Consequently, as constant a ratio as possible of fibrematerial and of plastic material in the finished fibre-reinforcedplastic moulded parts can be maintained during the production of theseparts.

FIG. 2 shows a second embodiment of an injection moulding machine with asingle-screw plasticizing unit according to the invention. In contrastto FIG. 1, here the fibre material is not fed in the form of endlessfibres, but rather as cut fibres. Unlike in FIG. 1, the fibre materialand the plastic material are fed into the cylinder through the samebore. The cut fibres 35 are fed into the cylinder 4 via a gravimetricmetering device 50. Owing to the load cell 51, the ACTUAL mass flow ofcut fibres 35 is known. Consequently, here also as constant a ratio aspossible of fibre material and of plastic material in the finishedfibre-reinforced plastic moulded parts can be maintained during theproduction of these parts. Chopped glass fibres are preferably used ascut fibres.

FIG. 3 shows a single-screw plasticizing unit according to theinvention, which is operated without the addition of fibre material withonly one plastic component. In this case, only the volumetric meteringdevice 30 is used. By means of the method according to the invention, asconstant an ACTUAL mass flow of plastic material as possible can bemaintained. Thereby, it becomes possible that also in the case ofplastic material which is critical with regard to dwell time, thedesired ACTUAL mass flow is maintained. In particular, in a preferredembodiment the screw can be operated in an underfed manner. A preferredfield of application is the production of plastic moulded parts foroptical purposes.

Further variants of the invention are not illustrated. For example, itis also possible to work both endless fibres and also chopped glassfibres into the melt.

LIST OF REFERENCE NUMBERS

 1 injection moulding machine  2 clamping unit  3 single-screwplasticizing unit  4 cylinder  5 plasticizing screw  6 rotary drive  7linear drive  8 first filling opening  9 second filling opening 10a-10findividual fibre strands or respectively rovings 11 backflow barrier 12mixing part 13 fibre feed device 14 fibre storage container/fibre gate15 fibre spool 15a-15f fibre spools 16 antistatic tube 16a inlet opening16b outlet opening 17 deflector roller 18 brake roller 19 heatingelement 30 volumetric metering device 31 storage container 32 meteringscrew 33 rotary drive 34 granulate 35 cut glass fibres 40 fibre brakingdevice 50 gravimetric metering device 51 load cell

What is claimed is:
 1. A method for the production of plastic mouldedparts, with a plasticizing unit (3), which has a cylinder (4) and aplasticizing screw (5) able to be driven rotatably and linearly in thecylinder (4), wherein a plastic material (34) which is to be melted isfed into the cylinder (4) via an opening (8) in the cylinder (4),wherein the plastic material (34) is fed into the cylinder (4) by meansof a volumetric metering device (30), wherein the volumetric meteringdevice (30) has a storage container (31) for the receiving of plasticmaterial (34), a rotatable metering element (32) and a rotary drive (33)for actuating the metering element (32), and wherein molten plasticmaterial is injected into a moulding tool (2) by an injection stroke ofthe plasticizing screw (5), wherein the ACTUAL mass flow of the plasticmaterial (34) is calculated, wherein the ACTUAL mass flow is calculatedfrom the return speed v_(screw,back,n) of the plasticizing screw (5)during a melt metering process, the nominal diameter of the plasticizingscrew (5) and the melt density, that the ACTUAL mass flow of the plasticmaterial (34) is compared with a TARGET mass flow of the plasticmaterial (34), and that with a predeterminable difference value betweenthe ACTUAL mass flow of the plastic material (34) and the TARGET massflow of the plastic material (34) the rotation speed n_(d) of the rotarydrive (33) of the metering element (32) is changed in such a way thatthe difference value is reduced.
 2. The method according to claim 1,wherein fibre material is added into the cylinder (4) and afibre-reinforced plastic moulded part is produced, wherein endless fibrestrands (10 a, 10 b, 10 c, 10 d, 10 e, 10 f) are fed via a fibre brakingdevice and/or cut fibres (35) are fed via a gravimetric metering device(50) to the plasticizing unit (3), and wherein plastic material which ismolten and is mixed with fibre material is injected into a moulding tool(2) by an injection stroke of the plasticizing screw (5).
 3. The methodaccording to claim 2, wherein via a first opening (8) in the cylinder(4) the plastic material which is to be melted is fed as granulate (34)into the cylinder (4), that on the conveying remote side from the firstopening (8) the endless fibre strands (10 a, 10 b, 10 c, 10 e, 10 e, 10f) are fed via a second opening (9) and/or the cut fibres (35) are fedvia the first opening (8) or via a third opening into the cylinder (4)and are drawn in by the plasticizing screw (5) through rotation.
 4. Themethod according to claim 1, wherein the ACTUAL mass flow of fibrematerial on the feeding of endless fibres or on the feeding of cutendless fibres is calculated as follows:dm _(f) /dt=v _(f) *n _(f) *n _(tex) wherein dm_(f)/dt: actual mass flowof fibre material v_(f): fibre feed speed n_(f): number of fed fibrestrands n_(tex): thread fineness of a fed fibre strand
 5. The methodaccording to claim 1, wherein a change to the metering rotation speedn_(d) is carried out from injection moulding cycle to injection mouldingcycle, or that a PI controller is used for a change to the meteringrotation speed.
 6. The method according to claim 1, wherein the ACTUALmass flow of plastic material is averaged over several injectionmoulding cycles, and the thus formed mean value is used for a change tothe rotation speed n_(d) of the rotary drive (33) of the meteringelement (32).
 7. The method according to claim 1, wherein the ACTUALmass flow of plastic material is calculated as follows:dm _(k) /dt=D _(s) *π*v _(screw,back,n)*ρ_(s)(p,T)−dm _(f) /dt whereindm_(k)/dt: actual mass flow of plastic material D_(s): screw nominaldiameter v_(screw,back,n): screw return speed during the plasticizingρ_(s)(p,T): melt density p: pressure T: temperature dm_(f)/dt: actualmass flow of fibre material
 8. The method according to claim 1, whereinfor an injection moulding cycle the ACTUAL mass flow of the plasticmaterial is calculated and that for one of the subsequent injectionmoulding cycles, for the injection moulding cycle immediately followingtherefrom, an adaptation of the ACTUAL mass flow to the TARGET mass flowis carried out by changing the rotation speed n_(d) of the rotary drive(33) of the metering element (32).
 9. The method according to claim 1,wherein a metering screw (32) or a metering disc is used as meteringelement.
 10. The method according to claim 1, wherein the endless fibrestrands (10 a, 10 b, 10 c, 10 d, 10 e, 10 f) are withdrawn from a fibregate (18) equipped with fibre spools (15).
 11. The method according toclaim 1, wherein the fibre braking device (40) imparts an adjustable,slip-free speed to the fibre strands (10 a, 10 b, 10 c, 10 d, 10 e, 10f).
 12. The method according to claim 1, wherein the cut fibres are fedas chopped glass fibres or as a component of a plastic granulate. 13.The method according to claim 1, wherein the plastic material (34) whichis to be melted comprises at least one of granulate, powder, bars and/orliquid silicone.
 14. The method according to claim 1, wherein thedifference value is reduced to zero.